1. Field of the Invention
This invention relates to a signal amplification system producing an amplified signal with reduced distortion.
2. Description of Related Art
An ideal power amplifier amplifies an input signal with no waveshape alteration. The ideal power amplifier is therefore characterized as having a transfer function (input signal vs. output signal) which is linear with no transfer function discontinuities. In practice, a power amplifier, however, has a transfer function with nonlinear and xe2x80x9clinearxe2x80x9d regions. For the power amplifier to achieve as near to linear operation as possible, the power amplifier is designed to operate within its linear region given the range of possible input signal amplitudes. If the input signal has an amplitude which causes the power amplifier to operate outside the linear region, the power amplifier introduces nonlinear components or distortion to the signal. When the input signal possesses peak amplitudes which cause the amplifier to compress, to saturate (no appreciable increase in output amplitude with an increase in input amplitude) or to shut-off (no appreciable decrease in output amplitude with a decrease in input amplitude), the amplifier produces an output signal that is clipped or distorted in a nonlinear fashion.
In wireless communications systems, high power amplification of signals is used to increase the power of the signal to be transmitted, for example carrier signal(s) with information modulated thereon. The distortion of the input signal causes power to be generated in adjacent channels or frequencies to corrupt or interfere with signals in the adjacent channels or frequencies, commonly referred to as spectral regrowth or adjacent channel power (ACP). The generation of adjacent channel power is of particular concern in wireless communications systems where signals being amplified are in adjacent channels or frequency bands. Wireless cellular communications systems comprise a number of base stations, geographically distributed to support transmission and receipt of communication signals to and from wireless units, which can be mobile or fixed, in the geographic region. Each base station handles voice and/or data communications over a particular region called a cell, and the overall coverage area for the cellular system is defined by the union of cells for all of the cell sites, where the coverage areas for nearby cell sites overlap to some degree to ensure (if possible) contiguous communications coverage within the outer boundaries of the system""s coverage area.
In a wireless cellular communications system, a base station and a wireless unit communicate voice and/or data over a forward link and a reverse link, wherein the forward link carries communication signals from the base station to the wireless unit and the reverse link carries communication signals from the wireless unit to the base station. There are many different schemes for determining how wireless units and base stations communicate in a cellular communications system. Multi-user wireless communications systems, such as Code division multiple access (CDMA), Time division multiple access (TDMA), Global System for Mobile Communications (GSM) and orthogonal frequency division multiplexing (OFDM), combine multiple voice and/or traffic channels into a single or multiple carriers. A linear amplifier should be able to react rapidly to transmit power changes and bursty traffic variations within the transient response specifications in the microsecond and millisecond ranges while providing adequate error cancellation. There is therefore a need to devise techniques that can eliminate substantially or reduce significantly the distortion produced by the amplifier.
Feed-forward correction is routinely deployed in modern amplifiers to improve amplifier linearity with various input patterns. The essence of the feed-forward correction is to manipulate distortion, such as intermodulation (IMD) components, created by the amplifier so that at the final summing point, the distortion cancels out. Due to the unpredictability of input RF carrier pattern as well as the resultant distortion location, a known frequency component, i.e. a pilot signal, is injected in the main signal path with the distortion produced by the amplification process. In feed-forward amplifiers, the feed forward distortion reduction circuitry minimizes the pilot signal along with the distortion. As such, by designing the feed forward distortion reduction circuitry to detect and cancel the pilot signal, the distortion can also be removed.
The pilot signal is an electrical signal comprising at least one frequency component spectrally located within or near the frequency band of operation of the electrical circuit. A more complete description of the pilot signal is shown in FIG. 1 which shows the frequency response of a radio frequency (RF) amplifier including the location of the pilot signal. The pilot signal can be located near the lower edge of the operating band (e.g., pilot 1) and re-tuned to be located near the upper edge of the band of operation (e.g., pilot 2). The pilot is positioned a spectral distance of xcex94f from an edge of the band of operation whose center frequency is f0. The electrical characteristics (e.g., amplitude, phase response, spectral content) of the pilot signal are known. It should be noted that although the pilot signal is shown as having one or two spectral components, the pilot signal can be tuned to have more spectral components or be spread across the spectrum. The pilot signal is detected a spectral component at a time, and the spread spectrum pilot is de-spread and detected as a single amplitude for the spectrum.
The feed forward distortion reduction circuitry typically reduces distortion produced by the RF amplifier by applying the pilot signal to the RF amplifier and making adjustments based on information obtained from the applied pilot signal. FIG. 2 discloses feed-forward correction circuitry 10 and its use of information obtained from the pilot signal to reduce distortion produced by RF amplifier 12. An input signal, for example including at least one carrier signal with information modulated thereon, is applied to a splitter 14. The splitter 14 replicates the input signal on a main signal path 16 and a feed forward path 18. The splitter 14 is part of a carrier cancellation loop referred to as loop #1, which in addition to the splitter 14, comprises gain and phase circuit 20, coupler 22, the RF amplifier 12, delay circuit 24 and couplers 26 and 28. The signal on the main path 16 is applied to gain and phase circuit 20. The output of gain and phase circuit 20 and the pilot signal are applied to the coupler 22. Typically, the amplitude of the pilot signal is much less (e.g., 30 dB less) than the amplitude of the input signal so as not to interfere with the operation of the amplifier 12. The output of the coupler 22 is applied to the amplifier 12 whose output comprises the amplified input signal, the amplified pilot signal and distortion signals produced by the amplifier 12.
A portion of the output of the amplifier 12 is obtained from the coupler 26 and is combined at the coupler 28 via coupling path 30 with a delayed version of the input signal on the feed forward path 18 to isolate the pilot signal with distortion on the feed forward path 18. The input signal on the feed forward path 18 is sufficiently delayed by delay circuit 24 so that such signal experiences the same delay as the signal appearing at the coupler 28 via the path 30. The resulting error signal contains the distortion produced by the amplifier 12 along with any portion of the carrier signal remaining at the output of the coupler 28 and the pilot signal. The amount of carrier cancellation in the carrier cancellation loop depends on the proper gain and phase match between the two paths from the splitter 14 to the coupler 28.
The gain and phase circuit 20 adjusts the phase and gain of the input signal according to control signals on control paths 32 and 33 such that the signal appearing at the coupler 28 via the path 30 is substantially the inverse (equal in amplitude but 180xc2x0 out of phase) of the delayed input signal at the coupler 28. The gain and phase control signals appearing on the control paths 32 and 33 of the gain and phase circuit 20 are derived from the signal at the output of the coupler 28 in a well known manner by sampling the output of the coupler 28 with a coupler 34 and using signal detection and control circuitry 35. In general, the signal detection and control circuitry 35 detects an error signal for the carrier cancellation loop. The error signal represents the amplitude of the signal at point A, and the signal detection and control circuitry 35 attempts to reduce the amplitude of the carrier signal(s) by providing gain and/or phase control signals.
In this embodiment, the signal detection and control circuitry 35 includes a detector 36, such as a log detector, which produces a signal representing the amplitude of the signal at point A. A filter 38 filters the output of the log detector to produce a DC-type amplitude signal representing the amplitude of the carrier signal(s). The amplitude signal is provided to a nulling circuit 40. In response to the amplitude signal, the nulling circuit 40 provides the control signals on the control paths 32 and 34 to adjust the relative gain and/or phase between the combining signals at the coupler 28 and reduce the carrier signal(s). When the carrier signal(s) is minimized, the carrier signals combined at the coupler 28 substantially cancel each other leaving at the output of the coupler 28 the pilot signal with distortion produced by the amplifier 12. Loop #1 is thus a carrier cancellation loop which serves to isolate on the feed forward path 18 the pilot signal with distortion produced by the amplifier 12.
A distortion reduction loop or loop #2 attempts to reduce the pilot signal on the main signal path 16, thereby reducing the distortion produced by the amplifier 12, using the signal at the output of the coupler 28. The pilot signal with distortion on the feed forward path 18 is fed to a gain and phase circuit 42. The output of the gain and phase circuit 42 is fed to amplifier 44 whose output is applied to coupler 46. The coupler 46 combines the amplified pilot signal and distortion on the feed forward path 18 with the signals from the amplifier 12 on the main signal path 16 (carrier signal(s), pilot signal with distortion). A delay circuit 40 on the main signal path 16 delays the signals from the output of the amplifier 12 on the main signal path 16 to experience substantially the same delay as the corresponding signals from the output of the amplifier 12 which pass over the coupling path 30 through the coupler 28 to the coupler 46.
A coupler 48 provides a signal representative of the signal at the output of the coupler 46 onto a pilot detection path 50. Because the frequency, amplitude and other electrical characteristics of the pilot signal are known, pilot detection and control circuitry 52 can detect the amplitude of the remaining portion of the pilot signal from the signal on the pilot detection path 50. The pilot detection and control circuitry 52 determines the amplitude of the pilot signal, and in response to the amplitude of the remaining pilot signal, the pilot detection and control circuitry 52 provides control signals to the phase and gain circuit 42. In general, the pilot detection and control circuitry 52 will detect the pilot signal and use this information to generate control signals onto paths 66 and 68 to cause the gain and phase circuit 42 to adjust the gain and phase of the pilot signal on the feed forward path 18 such that the pilot signal on the main path 16 as well as the distortion is substantially the inverse (equal in amplitude but 180xc2x0 out of phase) of the pilot signal and the distortion on the feed forward path 18 at the coupler 46. The corresponding pilot signals and distortion substantially cancel each other at the coupler 46 leaving the carrier signal(s) at the output of the system. Therefore, loop #2 is a distortion reduction loop which attempts to cancel the pilot signal to cancel substantially the distortion produced by the amplifier 12.
In this embodiment, the pilot detection and control circuitry 52 includes pilot receive circuitry 54 which includes a mixer 56 to frequency convert the error signal on the pilot detection path 52 to a lower frequency and a filter 58 to facilitate detection of the pilot signal by a signal detector 60 at the known frequency for the pilot signal. The detector 60, such as a log detector, produces a signal representing the amplitude of the signal the output of the coupler 46. A filter 62 filters the output of the detector 60 to produce a DC-type amplitude signal representing the amplitude of the remaining pilot signal. The amplitude signal is provided to a nulling circuit 64. In response to the amplitude signal, the nulling circuit 64 provides gain and phase control signals on the control paths 66 and 68 to the phase and gain circuit 42. The control signals are provided to adjust the relative gain and phase between the signals being combined at the coupler 46 and reduce the amplitude signal, thereby reducing the remaining pilot signal. The amount of cancellation of the pilot signal indicates the amount of distortion cancellation. When the amplitude of the pilot signal is minimized, the pilot signals and distortion combined at the coupler 46 substantially cancel each other at the output of the coupler 46.
In a pilot-based feed forward amplifier distortion reduction system, the amplitude of the pilot signal is typically relatively small at the output of the distortion reduction system because of the cancellation of the pilot and the relative amplitude of the pilot signal with respect to the amplitude of the output signal. Thus, it becomes difficult to detect the pilot signal at the output of the system. To improve detection of the pilot signal at the output of the distortion reduction system, schemes are developed to generate the pilot signal at an appropriate location and to improve detection and control. Such schemes typically add costs to the systems. Pilotless feed forward distortion reduction schemes have been developed to eliminate the pilot signal, thereby eliminating the need for the pilot generation, detection and control circuitry. The pilotless feed forward reduction systems, however, do not have a known pilot signal which can be more readily detected at the output of the feed forward distortion reduction system to compensate for changing operating conditions.
A limitation in the operation of feed forward amplifier distortion reduction systems involves the ability of the feed forward amplifier distortion reduction system to operate over a wide frequency range. Feed forward distortion reduction systems require tight operating tolerances, for example, typical feed forward correction systems may require a + or xe2x88x920.1 dB frequency flat response (amplitude deviation over the frequency band of operation) and a + or xe2x88x921 degree phase linearity (phase deviation in the frequency band of operation) to achieve sufficient and consistent performance over the frequency band. In general, a phase difference of 179 to 181 degrees and an amplitude difference of + or xe2x88x920.1 dB between the combining signals can achieve a cancellation of 30dB, and a 175-185 degree phase difference and a 2dB amplitude difference can provide almost 20 dB of cancellation. Equalizers in the main and feed forward paths of the feed forward amplifier have been used to improve the phase and amplitude flatness over frequency. Such equalizers are usually tuned in a position which yields desired performance for a given set of RF carriers and temperature and then remain fixed. However, once the temperature or RF carriers subjected to the amplifier have changed, the equalizer effectiveness deteriorates. Adjustable equalizers have been implemented using broadband power detectors as the sensor for adjustment. The broadband composite power is monitored in the feed forward amplifier architecture, and the equalizer is adjusted to minimize the composite power level detected. Because the detected power level or amplitude is over a broad frequency band, this technique can lead to non-uniform adjustments across frequency since a sloped frequency response could yield the same measurement as a flat one. Accordingly, as operating conditions change, the amplifier distortion reduction schemes can be vulnerable to amplitude and/or phase response changes in the amplifier architecture, especially amplitude and/or phase response changes over the frequency band of operation.
The present invention is an amplifier distortion reduction system that detects a plurality of amplitudes corresponding in time within a frequency band of operation. In response to at least one of the plurality of amplitudes, adjustments can be made to components within the frequency band of operation, enabling the amplifier distortion reduction system to adapt to changing operating conditions. For example, processing circuitry can use time samples to produce a frequency spectrum representation of the frequency band of operation. The processing circuitry can include an analog to digital (A/D) converter which samples and converts the radio frequency (RF) signals over the frequency band of operation into digital sample values. A digital receiver transforms the digital sample values to produce a digital domain frequency spectrum representation of the frequency band of operation, for example using a Fourier Transform. The frequency spectrum representation can be a plurality of amplitudes within the frequency band of operation which correspond in time, such as a plurality of amplitudes over frequency representing the frequency spectrum at a given instant in time. In response to the frequency spectrum representation, the digital receiver provides adjustments for components in the frequency band of operation to improve the performance of the amplitude distortion reduction system. In a feed forward embodiment, the output of the carrier cancellation loop can be monitored and equalizer adjustments provided to reduce the amplitude of the carrier signal(s) equally over the frequency band of operation. The output of the IMD cancellation loop can be monitored and equalizer adjustments provided to reduce the IMD components equally over the frequency band of operation. Thus, the processing circuitry can monitor amplitudes corresponding in time over the frequency band and provide improved performance over the frequency band.